During periods of extreme cold, heat pumps usually cannot provide enough heat to maintain the desired inside temperature. For this reason, it is common, particularly in areas that regularly have periods of sustained cold temperatures, to provide an auxiliary heat source. These auxiliary heat sources are typically an electric heater or a fossil fuel (e.g., gas) furnace. In the case of fossil fuel furnaces, it is undesirable that the heat pump and the furnace operate at the same time. The most common solution to preventing the heat pump and auxiliary fossil fuel furnace from operating simultaneously is to install a fossil fuel kit. However, fossil fuel kits are expensive and usually require installation of a separate control panel and at least two temperature sensors. The installer typically must set/adjust an outdoor temperature at which the compressor is locked and the auxiliary fossil fuel furnace is used instead. However, the proper temperature varies with the heat pump efficiency, home insulation, current weather conditions (e.g., sunny or cloudy) and the interior temperature set point.
The present invention relates to a control for a heat pump having an auxiliary heat source that operates the auxiliary heat source and locks out the heat pump based on the estimated heating load, rather than on directly sensed outside temperature. The estimate of heat load is based on the temperature relative the set point temperature and duty cycle of the load. The estimate may then be used to decide when to lockout the heat pump. This eliminates the need to install a fossil fuel kit, and in particular to install the outside temperature sensors typically included in such kits. The method of the present invention operates the auxiliary heat source and locks out the heat pump based on the relative time that either of the heat sources is xe2x80x9conxe2x80x9d and the time that both of the heat sources are xe2x80x9coffxe2x80x9d.
This can be conveniently implemented using a counter that increments when a heat source is xe2x80x9conxe2x80x9d and decrements when the heat sources are xe2x80x9coffxe2x80x9d (or vice versa). Thus the counter acts as a measures of the heat load, a high counter indicating that the heat sources have been xe2x80x9conxe2x80x9d relatively more time than they have been xe2x80x9coffxe2x80x9d, which it typically the result of unusually cold outside temperatures, and a low counter indicating that the heat sources have been xe2x80x9coffxe2x80x9d relatively more time than they have been xe2x80x9conxe2x80x9d. The controller turns xe2x80x9conxe2x80x9d the heat pump if there is a call for heat and the counter is below a first threshold, and turns xe2x80x9conxe2x80x9d the auxiliary heat source if there is a call for heat and the counter is above the first threshhold. The controller also turns xe2x80x9conxe2x80x9d the auxiliary heat if the counter reaches a second threshold before the demand for heat is satisfied. The control may delay turning xe2x80x9coffxe2x80x9d the heat pump after turning xe2x80x9conxe2x80x9d the auxiliary heat source, to allow it to continue to provide heat as the auxiliary heat source warms up.
The control and the control method of the present invention automatically take into installation-specific parameters such as heat pump efficiency and home insulation, as well as variable parameters, such as current weather conditions and inside temperature set point. Thus, the actual operation of the system is not dependent upon temperatures settings based upon estimates made at the time of the installation of the system, and automatically takes into account changes in conditions.